777
FXUS63 KLOT 190619
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
119 AM CDT Wed Jun 19 2024

.KEY MESSAGES...

- Hot and muggy conditions will prevail through the end of the
  week, although cooler conditions are expected near the lake
  Thursday and Friday.

- Periodic chances for thunderstorms will continue, though many
  hours will remain dry.

&&

.DISCUSSION...
Issued at 248 PM CDT Tue Jun 18 2024

Through Wednesday Night:

Forecast focus is on thunderstorm potential overnight through
Wednesday evening. A cold front will be moving into the
northwest cwa Wednesday afternoon. There may be some lingering
showers/isolated thunderstorms ahead of this front into far
northwest IL around daybreak Wednesday morning. As the front
moves further into the area Wednesday afternoon, at least
isolated thunderstorms will be possible. Models have a general
consensus of a narrow ribbon of instability along/ahead of this
front thus still some uncertainty for coverage with the best
chance for any thunderstorms generally along/north of I-80. Main
severe threat from any the storms would be localized damaging
winds. The cold front is also expected to accelerate down Lake
Michigan and several of the cams show winds turning
northeasterly off the lake by late Wednesday afternoon. This
could also serve as an area for development for isolated
thunderstorms.

Some of the models show a weak wave lifting north across
eastern IL and IN late tonight into Wednesday morning, that may
bring a few showers/isolated thunderstorms to the southeast cwa
by daybreak Wednesday morning.

Temperatures have warmed into the lower 90s for most areas, with
dewpoints dropping back into the mid/upper 60s this afternoon.
This will keep heat index values in the upper 90s to a few
locations tagging 100. Lows tonight will be similar to this
morning, mid 70s for most locations, warmer in Chicago.
Wednesday will be similar to today, with highs back in the
lower/mid 90s along with dewpoints in the mid/upper 60s with
heat index values again in the upper 90s to 100. With the
expected wind shift off the lake late Wednesday afternoon, could
see some temps dropping into the 70s for areas near the lake,
though overall confidence is low for the timing/speed of the
boundary. Lows Wednesday night/Thursday morning are expected to
be back into the upper 60s/lower 70s. cms


Thursday through Tuesday:

The amplified upper level ridge responsible for this ongoing
stretch of hot weather will be in the process of flattening out
heading into Thursday. This will bring the local area closer to
the northern extent of 90 degree temperatures. An associated
surface boundary is forecast to stall out somewhere near or just
northwest of the local area on Thursday. This will serve as the
focus for potential thunderstorm development, particularly
across northwest Illinois and southern Wisconsin Thursday
afternoon. A further south placement of this boundary would
result in increasing chances of showers and thunderstorms into
northern Illinois. For now opted to maintain 30-50% chances for
areas mainly along and west of the Fox River Valley and north
of I-80, though isolated storms can`t be ruled areawide (20%
chance). While there are differences in the magnitude of its
influence, onshore flow will keep temperatures cooler near the
lakeshore, and potentially further inland across northeast
Illinois if a backdoor front can make further inroads resulting
in high temperatures only in the upper 70s to mid 80s.
Otherwise expect 90s yet again for the rest of the area.

By Friday this boundary will begin to retreat back to the north
and closer to the lakeshore, keeping mainly lake adjacent areas
cooler. Accordingly, storm chances are relegated to near and
north of the WI/IL stateline, otherwise capping looks to keep
most areas dry. Saturday continues to look like the warmest day
for most areas late week, including near the lakeshore given
strong southerly flow developing in response to a developing
surface low across the Northern Plains. This will allow
temperatures to warm well into the 90s for all areas, including
near the lakeshore. This surface low will eventually swing a
cold front through the area Saturday night into early Sunday
morning bringing the areas best potential for showers and
thunderstorms (50-60% chance).

In the wake of this front cooler temperatures will prevail for
the start of the week, albeit still warmer than normal, with
highs in the mid-upper 80s. There are signs that this break will
be short lived, however, with 90s returning to the forecast
already by Tuesday as the upper ridge builds back across the
central CONUS.

Deep mixing each day this week will keep dew points mostly in
check this week with heat indices in the 90-100 degree range.
While no heat headlines are planned with this update, continue
to take steps to beat the heat this week!

Petr

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 119 AM CDT Wed Jun 19 2024

Key Messages:

- Westerly wind shift possible at RFD around 08-09Z. Low chance
  of a shower accompanying this wind shift as well.

- Chance for thunderstorms this afternoon.

- Northerly/northeasterly wind shift this evening with potential
  for MVFR ceilings afterwards.


A decaying line of convection is entering northwest Illinois at
press time. The associated precipitation probably won`t survive
to RFD (only about a 15% chance that it does), but even if it
does, notable visibility reductions are even less likely. The
outflow from this convection is more likely to reach RFD and
cause a westerly wind shift some time around the 08-09Z time
frame, though a return to prevailing southerly winds is favored
soon thereafter as the boundary lifts northward.

This afternoon, thunderstorms are favored to develop along this
remnant outflow boundary, and could also freely develop in the
muggy and uncapped air mass south of it. These storms will
likely be "pulse"-like in nature, collapsing soon after they pop
up with new storms developing along the outflow boundaries spit
out by the preceding storms. Thus, confidence is low in a storm
affecting any one TAF site. Convective coverage will likely be
greatest closer to the remnant outflow boundary, so think that
RFD stands a better chance at seeing storms this afternoon
compared to the Chicago metro TAF sites, but for the time being,
will let the inherited PROB30 groups for TSRA ride a while
longer in all five of our TAFs given the remaining uncertainties.

Lastly, a northerly/northeasterly wind shift is expected late
this afternoon or evening at all TAF sites. The timing of this
wind shifts remains somewhat uncertain as it is unclear whether
this wind shift will be caused by a true cold front later in the
evening or whether a composite outflow boundary serving as an
effective cold front will cause this wind shift to occur a few
hours earlier. Either way, after a brief uptick in winds in the
immediate wake of the boundary passage, winds should become
light and remain that way overnight. Behind the boundary, lake-
induced stratus may ooze inland and affect the Chicago metro
terminals, potentially resulting in MVFR ceilings being
observed for some time.

Ogorek

&&

.CLIMATE...
Issued at 305 AM CDT Tue Jun 18 2024

With a high temperature of 97 degrees recorded at O`Hare Airport
yesterday, Chicago broke its daily high temperature record of 96
degrees for June 17th, which was previously set in 1887 and
1957. A few additional temperature records (primarily some of
the record warmest low temperatures for both Chicago and
Rockford) may be threatened over the next few days.

Here are the daily high and warmest low temperature records
for Chicago and Rockford for each day through Saturday, June
22nd:

                Chicago
---------------------------------------
Day:           6/18 6/19 6/20 6/21 6/22
---------------------------------------
Record High:     98  102  104  101   97
Record Warm Low: 77   78   78   74   76

                Rockford
---------------------------------------
Day:           6/18 6/19 6/20 6/21 6/22
---------------------------------------
Record High:     99   99  101  100   97
Record Warm Low: 74   76   73   71   73

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
     INZ010-INZ011.

LM...Small Craft Advisory until 4 AM CDT early this morning for the
     IL nearshore waters.

&&

$$

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